The present invention is related to medical devices and systems, particularly therapeutic ultrasound systems.
Percutaneously introduced catheters having ultrasound transducers thereon can be used to deliver localized doses of therapeutic ultrasound energy to various sites within a body. Such systems are ideally suited for treating or preventing pathological conditions such as arterial restenosis due to intimal hyperplasia.
To achieve a high level of therapeutic effectiveness, a high amplitude of ultrasound vibration is required. Unfortunately, the acoustic output from a conventional transducer design is typically limited by the inherent properties of the piezoelectric material which forms the transducer. Specifically, when operating typical piezoelectric ceramic transducers at high vibrational amplitudes, the ceramic tends to fracture. This transducer failure is caused by the high tensile stresses within the ceramic material during transducer operation, and the problem is exacerbated by the fact that although piezoelectric ceramic materials tend to have high compressive strengths, they have relatively low tensile strengths.
The present invention provides ultrasound and other vibrational transducer systems comprising a vibrational transducer, typically an ultrasound transducer, which can be operated at very high vibrational amplitudes without failure. As such, the present invention provides systems to prevent the ultrasound transducer, which preferably comprises a ceramic piezoelectric material, from breaking apart at high amplitude operation.
The present ultrasound transducer system is ideally suited for use in a catheter based therapeutic ultrasound energy delivery system.
In a preferred aspect, the present invention comprises a piezoelectric ceramic ultrasound transducer having a restraint received therearound. The restraint is dimensioned or otherwise formed to have a structure which exerts a compressive pre-stress on the piezoelectric ceramic transducer element where the stress can be maintained during the operation of the transducer. Advantageously, the compressive pre-stress provided by the restraint operates to prevent tensile failure of the ceramic transducer at high acoustic output.
In a preferred aspect, the strength of the compressive pre-stress provided by the restraint on the transducer is approximately equal to the tensile strength of the transducer element. As will be explained, when this occurs, the restrained transducer can provide approximately twice the acoustic output of a comparable un-restrained device before tensile failure occurs.
In one exemplary aspect, the strength of the compressive pre-stress provided by the restraint is approximately half-way between the tensile strength and the compressive strength of the ceramic transducer material. As will be explained, when this occurs, the restrained transducer can be operated at a significantly increased output amplitude without failure.
In various preferred aspects, the compressive pre-stress provided by the restraint is just high enough to permit operation of the device without tensile failure at an output amplitude determined to be safe and effective for treating or preventing a pathological condition such as arterial restenosis due to intimal hyperplasia. In these preferred aspects, the required thickness and stiffness (as described below) of the restraint may be preferably kept to the minimum necessary to meet the acoustic output requirements, thereby minimizing the size of the device, and minimizing the requirements of the electrical drive circuitry, while maximizing the efficiency of the device in converting electric power into acoustic power.
In preferred aspects, the restraint may comprise a tensioned wire or filament(s) which is/are wrapped around the transducer. In other aspects, the restraint may comprise a jacket having an inner diameter which is initially fabricated to be slightly smaller than the outer diameter of the transducer. The jacket is then stretched to expand to a larger diameter such that it can just be received over the transducer. The transducer is then inserted within the expanded jacket, and the jacket is then allowed to contract such that it exerts a compressive pre-stress on the transducer. Systems for fabricating the jacket from a shape memory metal such as a nickel Titanium alloy (e.g.: Nitinol(trademark)) are also set forth.
The transducer is preferably cylindrically shaped, and may have an optional central longitudinal bore passing therethrough, with the bore defining an inner surface of the transducer. In various aspects, the inner and outer surfaces of the transducer are covered in whole or in part by an electrode. In alternative aspects, the opposite longitudinal ends of the transducer are covered in whole or in part by an electrode. In alternate embodiments of the invention, the transducer is formed from a series of alternating annular shaped polymer and piezoelectric ceramic rings, commonly referred to as a piezoelectric stack.
In a preferred aspect of the invention, the vibrational mode of the transducer is a relatively low frequency xe2x80x9cbreathing modexe2x80x9d, wherein the circumference of the cylinder oscillates around a nominal value, and the stress within the ceramic is predominantly in the tangential direction. In this case, tensile stress from the vibration of the transducer which may otherwise lead to failure can be balanced by compressive pre-stress in the tangential direction applied by a wrapped jacket type restraint.
In an exemplary aspect, the transducer may be made of a PZT-8, (or PZT-4) ceramic material, but other piezoelectric ceramics, electro-strictive ceramic materials, or non-ceramic materials such as piezoelectric crystals may be used as well.
In the aspect of the invention in which a wrapped restraint is used, the tensioned member wrapped around the transducer may be a metal wire, metal or polymeric braid, mono-filament polymer, glass fiber, or a bundle of polymer, glass or carbon fibers. Wires may have circular cross sections or be formed as a ribbon or square wire. In various aspects, the wire is placed under tension when initially wrapped around the ultrasound transducer so as to maintain the compressive pre-stress on the transducer. Alternatively, the tension may be introduced after the wrapping is applied using thermal, chemical, mechanical or other type of process.
Suitable materials which may be used for either of the wrapped or jacket-type restraints described herein include, but are not limited to, high tensile strength elastic material selected from the group consisting of steel, titanium alloys, beryllium copper alloys, nickel, titanium and other shape memory allows (e.g.: Nitinol(trademark)), and epoxy impregnated kevlar, glass, polyester or carbon fiber. In one exemplary embodiment of the invention, the restraint comprises a 0.001xe2x80x3xc3x970.003xe2x80x3 Beryllium Copper alloy ribbon wire having a tensile strength of 150,000 psi or greater, wrapped around the transducer under 0.25 lbs of tension.
In aspects of the invention where the restraint comprises a wire or ribbon wire, the restraint may comprise multiple layers of wire or ribbon wrappings using thinner ribbon or smaller wire than would be used for a single layer of wrapped restraint. An advantage of using such smaller diameter wire or thinner ribbon wire would be that reduced bending stress would be experienced during the wrapping process, thereby permitting the wire or ribbon to be tensioned to a higher average stress without breaking. This in turn would allow a higher compressive pre-stress to be applied to the ceramic transducer element using a thinner and less stiff restraint than would instead be required for a single layer wrap of the same material.
In those aspects of the invention where the restraint comprises a wire, ribbon wire, or other fiber under tension, the wire restraint may be fixed in place on the surface of the transducer by gluing, soldering or welding, with the compressive pre-stress being maintained during the operation of the transducer. Such fixation could be continuous or only at spaced apart points or regions along the contact length between the restraint and the transducer.
The use of a beryllium copper alloy wire as the restraint has numerous advantages including its high tensile strength, (typically 150 kpsi or greater), corrosion resistance and conductive properties. A further advantage is that a beryllium copper alloy wire is easily solderable. As such, it may be soldered both to an outer surface of the transducer, and between adjacent wraps around the transducer without the need for a special solder tab. In addition, a beryllium copper alloy wire can easily be soldered at temperatures below the Curie temperature of the ceramic transducer material, (which is about 300xc2x0 C. for PZT-8 ceramic). Typically as well, a beryllium copper alloy wire has a tensile strength / modulus of elasticity on the order of 190 kpsi/19 Mpsi={fraction (1/100)} advantageous ration is similar to that of stainless steel which typically has a tensile strength /modulus of elasticity on the order of 300 kpsi/30 Mpsi={fraction (1/100)}.
In the aspects of the invention where the restraint comprises a jacket, such jacket may be made from a very high strain limit material having good elastic properties and high tensile strength. Such a jacket could first be formed and then expanded to be slipped over the transducer and then allowed to recover, thereby radially compressing the transducer. If instead fabricated from Nitinol(trademark), the jacket can be formed and then expanded to be slipped over the transducer. If maintained at a sufficiently low temperature, the jacket will maintain its expanded size as it is placed over the transducer. When the temperature is allowed to rise above a critical value the jacket material will contract, thereby applying compressive pre-stress to the transducer.
In preferred aspects, a composite polymer is applied over the outside of the restraint. The composite polymer is adapted to dampen longitudinal axis vibrations, to provide an electrical insulating layer and to provide a convenient surface to which an outer jacket of the catheter may be attached. Suitable materials for such a composite polymer include, but are not limited to, materials selected from the group consisting of high strength adhesives such as epoxy or cyano-acrylate, and polymers such as heat-shrinkable PVDF, polyester, nylon, Pebax, PVDF or polyethylene.
The present invention also provides methods of generating and delivering high levels of therapeutic ultrasound energy to a patient. In particular, the present invention provides methods of delivering a high output from a therapeutic ultrasound energy delivery system by exerting a compressive pre-stress on a piezoelectric ceramic ultrasound transducer with a restraint wrapped or formed to be disposed around the transducer; and by maintaining the compressive pre-stress on the transducer during the operation of the transducer. In various aspects, the exertion of a compressive pre-stress on the ultrasound transducer is achieved by wrapping a tensioned wire or fiber(s) around the transducer. In other aspects, exerting a compressive pre-stress on the ultrasound transducer is achieved by expanding a jacket to a diameter sufficient to be received over the transducer, inserting the transducer into the jacket and allowing the jacket to contract against the outer surface of the transducer, or by fabricating the restraint from a shape memory material such as Nitinol(trademark) expanded to fit over the transducer and then shrunk with heat to apply a compressive pre-stress to the transducer.
In preferred aspects, the ultrasound transducer is cylindrical in shape and may further comprise a longitudinally extending bore therethrough. When air is disposed within this bore, the ultrasound energy emitted by the transducer will be directed predominately radially outwards, since very little ultrasound energy passes from the dense ceramic transducer into the low density air. Thus, the efficiency of the transducer can be enhanced, providing an ideal transducer system for mounting on a catheter.